Micrometer and nanometer copper particle-filled polyoxymethylene composites (coded as POM-micro Cu and POM-nano Cu, respectively) were prepared by compression molding. The compression strength and tensile strength of the composites were evaluated with a DY35 universal materials tester. An RFT-III reciprocating friction and wear tester was used to examine the tribological properties of the composites. The elemental compositions in the transfer films and the chemical states of the elements in the composite-worn surfaces were analyzed with electron probe microanalysis and X-ray photoelectron spectroscopy, while the surface morphologies were observed with scanning electron microscopy. It was found that Cu(-CH2-O-)(n) was produced in sliding of a POM-nano Cu pin against an AISI 1045 steel block and Cu2O was produced in sliding of a POM-micro Cu pin against the same counterface. POM-micro Cu exhibited higher copper concentration in the transfer film compared with POM-nano Cu, and the transfer film of the former was thick and patchy compared with that of the latter. It was also found that micrometer and nanometer copper particles as fillers in POM exhibit a distinctive size effect in modifying the wear mechanisms of the composites. In other words, the wear mechanism of POM-micro Cu is mainly scuffing and adhesion, while that of POM-nano Cu is mainly plastic deformation.